Resonant type shake table



1954 D M. ELLETT ET AL RESONANT TYPE SHAKE TABLE Filed Sept. 21, 1951 W m 1 EL X M 2 W/ T IVE 51553 Patented Aug. 17, 1954 UNITED STATES PATENT OFFICE RESONANT TYPE SHAKE TABLE Application September 21, 1951, Serial No. 247,572

Claims.

This invention relates to shake tables and, more particularly, it relates to an adjustable fre quency shake table capable of large excursions with small input energies.

It is standard practice in industry to test the durability of parts against vibrational fatigue by subjecting the parts to vibratory conditions similar to those which they will encounter in use.

Various types of shake tables have been developed in the past. The preponderance of apparatus in this class were developed for low frequency operation and were activated by eccentric mechanical devices.

It has been found that a shake table capable of suflicient amplitude at high frequency is best obtained by utilizing the principle of resonance in the table mounting and by use of an alternating current electromagnetic actuator. In accordance with the present invention, and utilizing these principles, it is feasible to fabricate shake tables capable of producing force equivalent to 30 G's at frequencies up to 300 cycles per second.

Accordingly, it is an object of the present invention to provide vibration test apparatus in which high amplitudes of motion are obtained at any selected frequency within a wide range of frequencies with a small amount of input energy.

' It is another object of this invention to provide a shake table of the resonant type in which the natural period of vibration can be adjusted.

The above and other objects are accomplished by supporting the shake table on a plurality of resilient supporting legs. One end of each of the legs is aflixed to the table and the other end of each of the legs is fixed to a foundation. The length of the legs and, accordingly, the resonant frequency, is varied by sliding up or down a bridge member which closely surrounds the legs and is immovably supported against lateral motion.

The table itself is caused to move to and fro, or vibrate, by an electromagnetic motor, the energization of which is adjustable in frequency. In fact, as is well known in the art, electromagnetic actuators are available which automatically adjust their frequency to the resonant frequency of the table.

It follows that the table is supported on cantilever bars. An appropriate frequency formula can be used to give the approximate length of the cantilever bar for any frequency so that by setting the bar for a desired frequency and hunting with the oscillator, the resonant frequency can be located. Then, by regulating the energy output of the electromagnetic motor, the desired accelerations can be built up. In the present invention, by knowing the weight of the table with a load and without a load, and by use of the appropriate spring and stress formulae, a cantilever type leg is designed which will resonate at any frequency within the design range, if adjusted to the correct length, and will not exceed the fatigue strength of the material of the legs with an acceleration of 30 G's on the table and its load.

The problem of leg design for the vibration table of the invention is simplified by assuming a leg with a given cross section and Working through for the length and stress at any frequency. It was found, as a general rule, that the frequency of the table varies inversely as the length of the leg to the three halves power; that, for given accelerations, the most severe design conditions occur when frequency is lowest and the leg is the longest; that, with respect to stress, a square leg is better than a round leg; that a rectangular leg is the optimum design if the vibration displacements are perpendicular to the wide side of the leg, and that the stresses in the leg decrease as their number and cross section are increased. The final design of the vibration table of this invention was achieved by consideration of all of the above factors.

The invention is best understood by reference to the following drawing which shows an isometric view of the vibration table of the invention. The platform for holding the part to be tested is designated by numeral l0. For supporting the platform, four resilient legs I I are rigidly attached to supporting structure I 2 which is rigidly supported by mounting on a concrete base. A center column I6 is likewise solidly afiixed to supporting structure l2.

For imparting vibration to the resilient legs, and hence to the platform, four ganged electromagnetic vibration units or vibrating means H, carried for example by generally U-shaped brackets I3, are attached to the supporting structure and coupled to the platform. The vibrating units I4 may be of any suitable type, for example, that shown at page 3 and described at pages 1-6 of Proceedings of the Society for Experimental Stress Analysis, vol. VI, No. II, copyright 1948, published by Addison-Wesley Press, Inc., Cambridge, Massachusetts. Other examples are the known electromagnetic Rollins shaker units, type 35, commercially available through the Rollins Company of Pasadena, California or those commercially available from The MB Manufacturing Company, Inc., New Haven, Connecticut, and described at page 8 of its bulletin No. 410A, copyright 1949. Such units have a normally stationary outer field casing which may be carried by the fixedly positioned U -shaped brackets l3 and also have an armature which may be connected or coupled to the table H1 or other suitable portions of the device by a stud, clamp or the like. An illustrative form of coupling means for the units [4 may comprise a stud or member 29 that connects with a movable armature portion 30 of a vibration unit [4 (the remainder of the particular vibration unit being broken away for clarity) and is screw threaded or otherwise attached at an opposite end thereto to a portion of the movable table II). In operation the armature vibrates longitudinally to and fro of the stator and sets into vibration the table or platform l coupled therewith. The pairs of units I4 shown at opposite ends of the table I0 are operated so that they cooperate in moving the table, e. g., those at one end may operate 180 electrical degrees out of phase with those at the opposite end so that one pair may exert a push against the table while the opposite pair exerts a pulling force, and conversely; this operation continues so long as the gauged units are energized. For adjusting the effective vibration length of the legs to the desired resonant frequency, a heavy bridge plate 15 having holes in each corner to receive the legs is slideably mounted on center column IS. The legs and center column pass through the openings in the bridge plate with a snug sliding fit. The bridge plate [5 is slideably adjusted in the vertical direction by means of two screw-threaded rods 18 which engage it in screw-threaded relationship. The rods I8 are journaled at their upper ends in a portion 17 of a frame, which also includes an additional portion 20 that may be fixedly secured in any appropriate manner to a stationary crossbar l9; this construction tends to insure that the column IB and threaded rods I8 are held against movement with the table 10 and rods II when the latter are actuated by the vibration means M. The table In and rods H are, of course, not fixed to the column I6 or frame portion 20 but are free of restraint longitudinally of the crossbar I9; if desired, guide means 23 may be carried by portions of the table structure and move therewith along the crossbar 19 to minimize table movement transversely of the crossbar. At their lower ends the rods iii are provided with spur gears 2| in mesh with a large driving gear 22 which is rotatably journaled on the center column IS. The driving gear 22 is journaled on column [6. The driving gear 22 is driven by electric motor 25, mounted on support means 21, through a belt drive as shown at 26. Adjustment of the effective vibration length of the legs to produce their resonant frequency is accomplished by moving the brdige plate l5 up and down through the operation of motor 25.

An advantage of the invention is that large accelerations are produced with small input energies by the utilization of the principle of vibrating a resiilent member at its resonant frequency.

Another advantage of the invention lies in the capability of vibration apparatus to be readily adjusted for operation at any desir d resonant frequency, and this adjustment can be accomplished while the machine is operating.

Further advantages of the invention are that it provides a means for subjecting heavy parts to relatively high frequency vibrations, and the apparatus itself includes means for automatically adjusting its natural vibration frequency to correspond to any selected test frequency.

While there has been disclosed a preferred embodiment of the invention, it will be apparent to those skilled in the art that the invention is not necessarily limited thereto, but that various modifications may be made without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

i. A device of the character described comprising the combination of a rod member, means adjacent one end portion of the rod member to anchor said end portion against movement in any direction, means adjacent and operatively connected with an opposite end portion of said rod member to vibrate said opposite end portion in a direction transverse of the rod length, and means to vary the effective length of said rod member including a bridge member engaging said rod member and slidable lengthwise therealong toward and away from said opposite end portion of the rod member, a substantially rigid guide member slidably engaging and retaining said bridge member against transverse movement, a rotatable threaded shaft generally parallel to said rod member and in engagement with a complementary portion of said bridge member, and means adjacent said one end portion of the rod member for rotating said threaded shaft to move the bridge member along said rod member and guide member.

2. A device of the character described comprising the combination of a plurality of generally parallel rod members, means adjacent one end portion of each rod member to anchor said end portion against movement in any direction, a platform secured to opposite end portions of the rod members adapted to support an object to be vibrated, means operatively connected with said opposite end portions of said rod members to vibrate said opposite end portions and platform in a direction transverse of the rod lengths, and means to simultaneously vary the effective lengths of said rod members including a bridge member engaging each of said rod members and slidable lengthwise therealong toward and away from said opposite end portions of the rod members and a substantially rigid guide member generally parallel to said rod members engaging and retaining said bridge member against transverse movement.

3. A device as claimed in claim 2 in which the means to vary said rod member effective lengths includes at least a single threaded shaft disposed generally parallel to said rod members and in threaded engagement with said bridge member, and means to rotate said threaded shaft to slide the bridge member along the rod members.

4. A device as claimed in claim 2, in which the means to vibrate said rod members is of the adjustable frequency type.

5. A device of the character described comprising the combination of a generally rectangular platform adapted to support an object, rod members secured to said platform adjacent corner portions thereof, means adjacent the opposite end portion of each rod member to anchor said end portion against movement in any direction, a plurality of means operatively connected with said platform to vibrate said platform and adjacent portions of the rod members in a direction transverse of the rod lengths, and means to vary the effective lengths of said rod members including a bridge member engaging each of said rod members and slidable lengthwise therealong toward and away from said platform and a substantially rigid column member generally parallel to said rod members engaging and retaining said bridge member against transverse movement.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,583,877 Hahemann et al. May 11, 1926 Number Number Name Date List Nov. 5, 1940 Bourne Oct. 21, 1941 Case Mar. 31, 1942 Dyer Dec. 14, 1943 Gross Oct. 31, 1944 Lazan Nov. 1, 1949 Tyler et al. May 15, 1951 FOREIGN PATENTS Country Date Great Britain July 18, 1951 

